The present invention relates to process control systems. More specifically, the present invention relates to a field transmitter having a normal operating or measurement mode and a diagnostic self-test mode.
A field transmitter is a transducer that responds to a measured process variable with a sensing element and converts the variable to a standardized transmission signal (e.g., an electrical or optical signal) that is a function of the measured variable. The term xe2x80x9cprocess variablexe2x80x9d refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH, and other properties.
Field transmitters are typically used to monitor process variables and measurements back to a control room in chemical, petroleum, gas, pharmaceutical, and other fluid processing plants. Often, these field applications are subject to harsh and varying environmental conditions.
All electrical components, including the physical sensors and the signal processing circuitry of field transmitters, have the potential to malfunction or fail. Such a malfunction could result in a faulty measurement being sent by the field transmitter to the control room. In addition, circuit drift can also impair the reliability of the transmitter. Routine testing by a skilled technician can detect problems with a transmitter, but that requires the technician to physically access the transmitter. In facilities having a substantial number of distributed field transmitters, there is a practical limit of how frequently a technician can visit and test each transmitter.
The present invention is a field transmitter for transmitting signals representative of process variables. The field transmitter has a physical sensor for sensing a process variable and generating a physical sensor signal which is representative of the process variable. The field transmitter also has a surrogate sensor for producing a surrogate sensor signal which is independent of the process variable. A signal processing circuit is included which converts either the physical sensor signal or the surrogate sensor signal to a measurement value. The field transmitter has a normal operating mode and a diagnostic self test mode. The physical parameter sensor is connected to the signal processing circuit during the normal operating mode, and the surrogate sensor is connected to the signal processing circuit during the diagnostic mode. If the measured value during the diagnostic mode is not the expected value produced by the surrogate sensor, the transmitter detects and can report a malfunction.